Temporomandibular (TM) disorders include a number of related painful conditions affecting the jaw. Causes are unknown. Epidemiological studies have limited value, but there is reason to believe that the incidence of oral-facial pain in the U.S. is high (Casey et al. 1979). Past pain research in the trigeminal system has focussed primarily on sensory mechanisms; little is known about the motor response to experimental and clinical pain. Rather than attempting to address causal issues in the clinical material, we propose to study the effects of experimental pain of specific and known origin on jaw motor function. We will then look for similar motor events in the clinical material. The goal of this research is to obtain information about the effect of pain on jaw motor function. We aim to arrive at an experimental model for the study of pain-modulated motor function which includes similar behavioral features as observed in situations of clinical pain. We will develop, refine and standardize a reliable model which is based upon the controlled, repeated injection of hypertonic sodium chloride solution into selected jaw muscles and the assessment of the perceived pain by means of visual-analog-scales (VAS). In healthy subjects, in quasi- static and dynamic tasks, using repeated measures design, we will study changes in postural activity and well-controlled jaw motor functions before pain induction, in pain and after pain has subsided. Experiments in the clinical population will involve the same tasks, including functional assessment before and after pain control, whenever possible. We will investigate the myoelectric activity in the painful muscle, its synergists and antagonists and quantify the altered jaw kinematics in pain. In the clinical material, we will test the hypothesis that no difference exists in painful function between acute muscle disorders and osteoarthritis. Finally, we will test whether or not the experimental and clinical pain have similar effects of jaw motor function, or in other terms whether or not the proposed experimental model includes similar features as found in disease.